Aroma compounds of natural origin are of major interest to flavour and fragrance industries, however, nature alone cannot meet the ever-increasing world demand on its own due to limited percentage of such compounds in plant kingdom. Therefore, there is a growing interest in developing alternative sources for natural aroma compounds and in particular, substituted 4-vinylphenols such as 4-vinylguaiacol (p-vinylguaiacol or 2-methoxy-4-vinylphenol or 4-hydroxy-3-methoxystyrene or 4-ethenyl-2-methoxyphenol), 4-hydroxystyrene (p-vinylphenol or 4-ethenylphenol), 3,5-dimethoxy-4-hydroxy styrene and others have been the most extensively investigated ones due to their widespread applications in food and alcoholic beverages, flavouring substances and as intermediates in the preparation of polymers and copolymers useful in coatings, electronic applications, ion exchange resins and photo resists etc. (Perfume and Flavor Chemicals, Aroma Chemicals, ed. Steffen, A., Allured Publishing Corporation. Vol I–IV (1994) and Encyclopedia of Food and Color Additives, ed. George, A. B., CRC Press, Inc., Vol I–II (1996)). The preparation of these substituted 4-vinylphenols such as 4-vinylguaiacol (FEMA GRAS No. 2675), 4-vinylphenol (FEMA GRAS No. 3739) and others are well known in the art, however, a more efficient process for preparing substituted 4-vinylphenols is desired and needed. The present invention provides a process wherein microwave assisted (Bose, A. K., Banik, B. K., Lavlinskaia, N., Jayaraman, M. and Manhas, M. S., Chemtech, 27, 18–24, (1997) and Larhed, M. and Hallberg, Drug Discovery Today, 6(8), 406–416, (2001)) condensation of substituted 4-hydroxy phenylaldehydes and malonic acid in the presence of organic base and organic acid provides only substituted 4-vinylphenols in one pot within 20 minutes and not cinnamic acid as generally obtained in conventional Knoevenagel-Doebner condensation reaction (Furniss, B. S., Hannaford, A. J., Rogers, V., Smith, P. W. G. and Tatchell, A. R: In: Vogel's Textbook of Practical Organic Chemistry, fourth Edn., ELBS, UK, 802 (1978); Susanne, R. H., Kerry, C. A., Dac, D. M., Ducan, J. N., Christopher, H. L., Rita, H. M., Mary, L. E., Nanette, N. F., Martin, S. W., Kjell, S. A., Matt, Z. J., Arvid, C. and Chiu-Hong, L., J. Med. Chem., 44, 4716–4732 (2001) and James, M., Jennifer, A. S. and Sonja, W., Tetrahedron Letters, 39, 8013–8016 (1998)). It is worthwhile to mention that microwave-assisted chemical transformation is a new emerging technique which is generally known for ecofriendly, rapid and high yielding process, however, such a surprising effect of microwave is observed for the first time in the above invention where both condensation and decarboxylation have occurred simultaneously without addition of decarboxylating agent.
The following prior art references are disclosed as below:    U.S. Pat. No. 6,468,566 discloses a method for the preparation of 4-vinylguaiacol from ferulic acid decarboxylase enzyme.    U.S. Pat. No. 6,235,507 disclose a method for the preparation of 4-vinylguaiacol from microbial conversion of ferulic acid at a pH more than 9.    U.S. Pat. No. 5,493,062 disclose a method for the preparation of 4-vinylphenol from p-alpha-aminoethylphenol (AEP) at high temperature.    U.S. Pat. No. 5,087,772 discloses a method for the preparation of 4-vinylphenol from 4-acetoxystyrene with a suitable alcohol in the presence of a suitable base.    U.S. Pat. No. 5,256,809 discloses a method for the preparation of 4-vinylphenol from 4-acetoxystyrene.
Journal of Biotechnology, (2000), 80, 195–202, discloses a method for the preparation of 4-vinylguaiacol from decarboxylation of ferulic acid by Bacillus coagulans. 
Enzyme and Microbial Technology, (1998), 23, 261–266, discloses a method for the preparation of 4-vinylguaiacol from decarboxylation of ferulic acid by Bacillus pumilus. 
Archives of Biochemistry and Biophysics, (1998), 359(2), 225–230, discloses a method for the preparation of 4-vinylphenol from decarboxylation of hydroxycinnamic acid by Klebsiella oxytoca. 
J. Fermentation and Bioengineering, (1996), 82(1), 46–50, discloses a method for the isolation of 4-vinylguaiacol from distilled and stored model solutions of “shochu” (a name of alcoholic beverage in Japan).
Encyclopedia of Food and Color Additives, ed. George, A. B., CRC Press, Inc., Vol II, 1705 (1996) discloses a method for the preparation of styrene by reaction of phenylaldehydes with acetic anhydride in the presence of sodium acetate to give cinnamic acid followed by decarboxylation of cinnamic acid.
Perfume and Flavor Chemicals (Aroma Chemicals), ed. Steffen, A., Allured Publishing Corporation, Vol II, 1891 (1994) discloses a method for the preparation of vinylphenol (4-hydroxy-3-methoxystyrene) by catalytic oxidation of 1,1-diphenylethane (1,1-di-(4-hydroxy-3-methoxy)phenylethane).
Journal of Biol. Chem., (1993), 268, 23954–23958, discloses a method for the preparation of 4-vinylguaiacol from decarboxylation of ferulic acid by Rhodotorula rubra. 
Appl. Environ. Microbial., (1993), 59, 2244–2250, discloses a method for the preparation of 4-vinylguaiacol from decarboxylation of ferulic acid by Saccharomyces cerevisiae and Pseudomonas fluorescens. 
Journal of Biol. Chem., (1962), 237, 2926–2931, discloses a method for the preparation of 4-vinylphenol from decarboxylation of 4-hydroxy-cinnamic acid by Aerobacter.
Journal of Biol. Chem., (1961), 236, 2302, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using pyruvate decarboxylase enzyme.
Journal of Biol. Chem., (1957), 227, 151, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using oxalate decarboxylase enzyme.
Journal of Biol. Chem., (1960), 235, 1649, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using glutamate decarboxylase enzyme.
Journal of Biol. Chem., (1957), 226, 703, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using aconitate decarboxylase enzyme.
Journal of Biol. Chem., (1964), 239, 879, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using aspartate 4-decarboxylase enzyme.
Tetrahedron Letters, (1999), 40, 6595–6598, discloses a method for the decarboxylation of trans-cinnamic acids into styrene derivatives by using plant cell cultures. Journal of Biol. Chem., (1962), 237, 2926–2931, discloses a method for the decarboxylation of trans-4-hydroxycinnamic acid into 4-hydroxystyrene.
Applied Catalyst A: General, (1995), 133, 219–239, discloses a method for the preparation of styrene from dehydrogenation of ethylbenzene.
Organic Synthesis Collective Volume I, 441–442 (1941) as well as Volume IV, 731–734 (1963), discloses a method for the preparation of styrenes by decarboxylation of cinnamic acids with quinoline in the presence of copper powder at 200–300° C. Some of other typical prior art references include U.S. Pat. Nos. 4,316,995; 4,868,256; 4,868,257; 4,933,495; 5,072,025; 5,128,253; 5,247,124; 5,344,963; 5,563,289; 6,111,133; European Pat. Nos. 0-128-984; 0-108-624; Dutch Pat. Nos. 72.09426; 72.13842; 75.04532; Japan Pat. Nos. 10306126; 6049137; J. Am. Chem. Soc., 70, 2295, (1948); J. Am. Chem. Soc., 72, 5198 (1950); J. Am. Chem. Soc., 80, 3645 (1958); J. Org. Chem., 23, 544–549 (1958); Chem. Berichte, 92, 2958–2961 (1959): Tertrahedron, 31, 235 (1975); Can. J. Chem., 63, 153 (1985). Although, the above methods have been proven to be useful, they suffer from one or more process deficiencies. For example, in some instances processes of this type necessarily involve resort to sub-ambient temperatures, which of course, involves some considerable process control and lead to reaction mixtures.
It, therefore, becomes an object of the invention to provide rapid and economical process for the preparation of substituted 4-vinylphenols from cheaper and commercially available 4-hydroxy phenylaldehydes as well as to eliminate the disadvantages associated with the above patents and papers.
In conclusion, the present invention discloses a simple and economical process for preparing vinylphenols starting from relatively cheaper and economical material 4-hydroxyphenylaldehydes and malonic acid in the presence of organic acid and organic base under microwave condition. Other objectives and advantages of the present invention will be apparent as the description progresses.